Parameterization of drag and turbulence for urban neighbourhoods with trees

Urban landscapes are replete with obstacles to air flow, both constructed and natural, and of multiple dimensions, shapes, and porosities. However, urban canopy parameterizations designed to be coupled with mesoscale models require knowledge of the neighbourhood-scale average, or integrated, effect of these obstacles at each height in the canopy without resolving each obstacle. To assess these neighbourhood-scale effects on the flow for simplified geometries, the results of microscale simulations may be scaled-up, i.e., horizontally-averaged. Here, obstacle-resolving Computational Fluid Dynamics (CFD) simulations of neutral flow through canopies of blocks (buildings) with varying distributions and densities of porous media (tree foliage) are conducted, and the spatial-average impacts on the flow of these building-tree combinations are assessed. Specifically, the drag and mutual sheltering characteristics, as well as the mutual impacts on the turbulent environment, are related to building plan and leaf area densities at each height. These relations then form the basis for a parsimonious parameterization of neighbourhood-scale drag and turbulence for treed urban canopies.